Liquid absorbent resins are able to absorb water, blood and the like equal to several times their mass, and are used in sanitary articles, diapers and the like in the fields of medicine and hygiene, as well as in water sealing agents in the field of construction.
In particular, liquid absorbent resins used in sanitary articles and diapers are required to be high added value products that are able to adequately absorb blood and urine and prevent soiling of clothing and the like due to leakage. In order to prevent leakage of blood and the like in sanitary articles, it is important to not only improve the liquid absorbency of the liquid absorbent resin, but also to maintain the liquid absorbent resin in a uniformly distributed state inside absorber such as the sanitary article, and various studies have been conducted for improving these. Here, an absorbent refers to a site that composes a portion of a sanitary article or diaper and is capable of absorbing blood and urine, and an example thereof is that a mixture of flocculent fibrous pulp and particulate liquid absorbent resin is enveloped in a non-woven fabric.
In recent years, in order to facilitate ease of production, studies have been conducted on ways to produce liquid absorbent resins by so-called water droplet-in-oil type, reverse phase suspension polymerization in which an aqueous solution containing a radical polymerizable compound and polymerization initiator is dropped into an organic solvent, the aqueous solution is dispersed in the organic solvent by stirring, and the radical polymerizable compound is polymerized in the resulting water droplets. However, since the liquid absorbent resin obtained according to the aforementioned production method is normally in the form of particles having a smooth surface, it is difficult to load them into flocculent fibrous pulp inside an absorbent, resulting in causing uneven distribution of the particles. As a result, such an absorbent is unable to adequately absorb blood and the like and tends to cause the occurrence of leakage. In addition, since a liquid absorbent resin obtained according to the aforementioned production method requires dust control measures and the like since the diameter of the resulting particles is extremely small, there are also problems with ease of handling.
As a way of resolving this problem, it has been reported that a highly absorbent resin is able to be produced that has a high degree of resin immobilization, that is, makes it difficult for the resin to become unevenly distributed within the absorbent, by suspension polymerization in which an aqueous solution containing a specific phosphate ester-based dispersant and a water-soluble vinyl monomer is gradually supplied into a hydrophobic organic solvent containing a nonionic surfactant (refer to, for example, Patent Document 1).
However, since the aforementioned highly absorbent resin did not have adequate affinity for blood and urine, it was not adequate for use as a sanitary article and the like in terms of liquid absorbency, and particularly blood absorbency. In addition, the aforementioned highly absorbent resin also had the problem of a significant decrease in liquid absorbency in the case of being made to repeatedly absorb blood and the like.
Moreover, an absorbent resin that is obtained by supplying an aqueous solution containing a specific phosphate ester-based surfactant, a specific polyacidic amino acid, an ethylenic unsaturated compound and a crosslinking agent into an inert solvent containing a specific phosphate ester-based surfactant to conduct suspension polymerization, and performing surface crosslinking treatment on polymer particles obtained by the suspension polymerization, has been reported to have extraordinarily superior absorbency characteristics with respect to blood (refer to, for example, Patent Document 2).
However, although an absorbent resin obtained by using polyaspartic acid for the aforementioned specific polyacidic amino acid has superior blood absorbency, it has the problem of having a brown color.
Products in the fields of medicine and hygiene, such as sanitary articles and diapers, in which liquid absorbent resins are used, are especially required to be clean and sanitary, and the presenting of a brown color by the aforementioned liquid absorbent resin can be said to be a serious problem since it causes the loss of a sense of cleanliness and gives an unsanitary impression, thereby significantly reducing the value of industrial use in said fields.
[Patent Document 1]
Japanese Unexamined Patent Application, First Publication No. 2001-31704
[Patent Document 2]
Japanese Unexamined Patent Application, First Publication No. 2003-206324